The present invention relates to the fabrication of optical waveguiding structures in lithium niobate.
Quasi-phase-matched (QPM) wavelength conversion devices based on lithium niobate waveguides have been widely studied for many years. The main applications of this technology include telecommunication systems, nonlinear optics, and blue laser sources for next generation DVDs [1, 2, 3]. QPM utilises periodic inversion, or poling, of the domain structure of lithium niobate to enhance the phase-matching capability of the material. The poled structure is commonly referred to as a grating. Lithium niobate poled in this way is known as periodically poled lithium niobate (PPLN). Several methods are now widely used for fabricating waveguides in this material, the most popular of which are annealed-proton exchange (APE) [4, 5, 6], titanium diffusion, and ion implantation. However, each of these developed techniques has some limits of applicability [7].
The fabrication of waveguides on PPLN substrates generally involves two steps in a sequence that depends on the choice of waveguide formation technology. For example, proton exchange involves firstly poling the lithium niobate (LiNbO3) with a certain period of grating and then forming a channel waveguide, whereas titanium diffusion reverses this order. APE waveguides are formed at relatively low temperatures (350° C.-400° C.) [5, 6], so normally the waveguides are fabricated after the lithium niobate sample has been poled. Poling after the waveguide formation results in a poor periodic grating structure. APE waveguides show increased resistance to photorefractive damage, but support only extraordinary guided modes [5]. In addition, proton exchanged layers decrease the nonlinear coefficient of the lithium niobate in the initial proton exchange layer [6], requiring complex post annealing to recover the nonlinearity. An alternative low temperature technique, ion implantation, requires the use of ion accelerators and so is complex and expensive.
The normal temperature for titanium diffusion into LiNbO3 is around 1050° C. to 1100° C. [7]. This process is used to fabricate titanium-diffused LiNbO3 waveguides to be used in conventional (non-QPM) optical components, such as optical modulators, which demonstrate good electro-optic properties, low propagation losses, and support both TE and TM guided modes [7]. However, the diffusion process is not compatible with periodically poled materials, because at such high temperatures the periodically switched domain structure is degraded. The alternative sequence process of poling after the formation of waveguides has been used with some success, but the formation of an unwanted thin domain inverted layer during the high temperature process for titanium diffusion may cause problems in the subsequent poling. An additional weakness of this technique is the worsened photorefractive damage in LiNbO3 induced by the incorporation of Ti4+ ions, which limits the operation of the Ti:LiNbO3-based devices to the infrared and visible range of the spectrum [7, 8]. Several methods for suppressing out-diffusion have been proposed, for example surface polishing off the 50 nm out-diffused layer after thermal processing [8], but those steps add complexity to the technique. In addition, it is very difficult to provide a uniform PPLN structure through a titanium in-diffused waveguide due to the electrical insulating properties of the lithium out-diffused layer in the waveguide area [7, 8].
To overcome such difficulties, low temperature diffusion (below 1000° C.) is desirable, requiring the use of elements with a high diffusion coefficient and a low activation energy. In this case, zinc appears to be suitable. The formation of low loss optical waveguides in LiNbO3 by zinc diffusion from ZnO surface films has been demonstrated [9, 11]. There is also a report on the fabrication of zinc-diffused waveguides in y-cut LiNbO3 for 1.32 μm wavelength operation by diffusion of surface metallic zinc [10]. Zinc-diffused waveguides in PPLN substrates grown by the Czochralshi method (where the PPLN domains are formed into the LiNbO3 crystal as it is grown) have also been reported, made by zinc diffusion from the vapour phase at low temperatures [12, 13]. Czochralshi-grown PPLN is limited in terms of quality and commercial viability, however. None of these techniques are well-developed.